1. Field of the Invention
The present invention relates to a semiconductor laser device and a method for fabricating the same, and, more particularly, to a semiconductor laser device which can be integrated with other semiconductor elements and a method for fabricating the same.
2. Brief Description of the Prior Art
In a conventional semiconductor laser device consisting of a laser medium which provides gain and a resonator structure which provides a feedback necessary for a build-up of laser oscillation, the resonator structure is composed of at least two plane surfaces which are parallel to each other and are usually formed by cleaving the laser medium, such as GaAs, GaP, etc.
This kind of semiconductor laser, however, has drawbacks such that an integration with other semiconductor elements, such as transistors, diodes, optical waveguides, etc. in one semiconductor body becomes very difficult, since the cleavage technique should be introduced to form the cavity resonator; and that a control of a wavelength generated therefrom becomes difficult, since the wavelength is determined by the length of the resonator which is defined by cleaved surfaces of the laser medium and it is very difficult to provide an accurate distance between cleaved surfaces.
In the field of semiconductor laser devices, it, therefore, has been desired to provide a semiconductor laser device which can be integrated with other semiconductor elements and the wavelength of which can be easily controlled.
There has been proposed a new type of a semiconductor laser device which is called a distributed feedback laser and which is capable to satisfy the above-mentioned needs ("Applied Physics Letters," Vol. 18, No. 4, February 1971, pp. 152- 154), and this semiconductor laser device has been developed and reported in "Applied Physics Letters", Vol. 22, No. 10, May 1973, pp. 515- 516, and "Applied Physics Letters", Vol. 23, No. 5, September 1973, pp. 224- 225.
The distributed feedback laser has a structure such that a surface of a laser active layer is periodically corrugated. The periods of the corrugations are determined by a desired laser wavelength to be emitted from the laser device, since a laser wavelength .lambda. of the laser device becomes EQU .lambda. = 2 S n/m,
wherein S is the spatial period of the corrugation, n is the effective refractive index of the laser active layer and m is an integer.
At the corrugated surface, however, there are many nonradiative recombination centers created during the fabrication of the corrugations at the surface of the laser active layer. Therefore, the distributed feedback laser has such a drawback that a threshold value for laser oscillation becomes high.
For solving such a drawback, there has been proposed a semiconductor laser device including a carrier confinement region, an optical confinement region and a corrugations disposed at an interface between the optical confinement region and a semiconductor layer having a lower refractive index than that of the material of the optical confinement region, to effectively lock in light within the optical confinement region. To be concrete, the semiconductor laser device comprises a GaAs semiconductor body of a first conductivity type having a major surface, a laser active region disposed on said major surface of said body, a GaAlAs semiconductor region of a second conductivity type, opposite said first conductivity type, and having a band gap wider than that of said laser active region, disposed on said laser active region, the surface of said GaAlAs semiconductor region opposite that disposed on said laser active region being corrugated, and a GaAlAs semiconductor layer of said second conductivity type, disposed on the corrugated surface of said GaAlAs semiconductor region and having a refractive index lower than that of said GaAlAs semiconductor region. In this concrete semiconductor laser device, the carrier confinement region consists of the laser active region, and the optical confinement region consists of the laser active region and the GaAlAs semiconductor region. This semiconductor laser device and its fabrication are described in detail in U.S. Patent Application, Ser. No. 512,969, assigned to the same assignee of this patent application.
This semiconductor laser device, however, has the drawback that the device is fabricated with a low yield rate, since, on the corrugations formed on the surface of the GaAlAs semiconductor region, the GaAlAs semiconductor layer is formed with a very low degree of reproducibility, and since, for obtaining the GaAlAs semiconductor layer on the corrugated surface of the GaAlAs semiconductor region with a high degree of reproducibility, there should be employed a molecular-beam epitaxial method which is described in detail in "Journal of Vacuum Science and Technology," Vol.8, No. 5, September/October 1971, pp. S31- S38, and "Applied Physics Letters", Vol. 25, No. 5, September 1974, pp. 288- 290, and which should be controlled with very high-degree epitaxial growth techniques.